The morphology and evolution of flare ribbons can also give information on the overall topology of the system. [1]} have shown that circular flare ribbons are associated with the presence of a null-point topology in the corona, while parallel ribbons moving away from each other have been interpreted as an indication of quasi-separator reconnection occurring higher and higher in the corona ([2]}). Different triggering mechanisms have been proposed ([3]}; [4]}; [5]}; [6]}; [7]}), but essentially the equilibrium of a magnetic FR embedded in an overlying magnetic field is determined by two competing effects: the outward-directed magnetic pressure between the FR and photosphere, and the inward-directed magnetic tension of the overlying field.
In the torus instability or catastrophic loss of equilibrium model ([3]}; [9]}; [10]}; [11]}) it is the onset of an ideal MHD instability that leads to the disruption of this equilibrium, while in the breakout model ([12]}; [13]}; [14]}; [15]}; [16]}) it is the onset of a resistive instability.
Assuming an overlying external field \(B_{ex}\)  that scales with the height \(z\)  from the photosphere as \(B_{ex} \propto z^{-n}\) , in the torus instability model the system becomes unstable when the apex of the axis of the magnetic FR reaches a critical height \(z_{cr}\)  where the decay index \(n\)  of the external overlying field \(B_{ex}\)  becomes larger than a critical value \(n_{cr}\) . The results of several MHD simulations place \(n_{cr}\)  in the range \([1.3-1.75]\)  ([17]}; [18]} [19]}; [20]}; [21]}; [22]}; [23]}; [24]}; [25]}). Attempt to estimate the decay index at the onset of solar eruptions have also been made both using limb observations as well as stereoscopic observations ([26]}; [27]}; [28]}; [29]}; [30]}). Contrary to the torus instability model that does not require any particular magnetic field topology, the breakout model requires a multi-flux distribution. The eruption begins when a resistive instability sets in at the breakout current sheet that exists between the arcade that confines the FR and the overlying field ([16]}). This reconnection removes the confining flux by transferring it to the neighboring flux domains. As a result, the magnetic tension of the confining field decreases resulting in an eruption. For the breakout model to work two conditions must be satisfied: the presence of a null-point or quasi-separator in the corona,
and the flux of the confining arcade must be larger than the flux of the overlying field.
Due to the nature of the problem, i.e., evidence of reconnection occurring higher up in the corona, observational studies that clearly support the breakout model are quite rare ([32]}; [33]}; [34]}; [35]}). Both models address the triggering of the eruption, but what determines if the eruption results in a CME or in a failed eruption? Many questions have to be answered, i.e.:
How does the trigger mechanism affect the eruptive/failed behavior of the flare? How important is the magnetic environment of the AR?
